An explanation of what this package is about

[This explanation is taken from the vignette accompanying the package.]

The probability calculus is an extension of the calculus of propositional logic. Besides the truth or falsity of propositions, the probability calculus also considers a continuum of degrees of credibility -- probabilities -- between those two extremes. Just as propositional logic gives a set of rules to derive the truth or falsity of some propositions from the truth or falsity of others, so the probability calculus gives a set of rules to derive the probability for some propositions from the probability for others; these rules include the logical ones as a special case. (See vignette for references.)

There are different logical deduction systems, each with many dialects, to express the rules of propositional logic. One of them is the sequent calculus. The probability calculus has many analogies with the sequent calculus.

In sequent calculus we express that proposition